fix: recover durable Gateway tasks by session

[EvanProgramming]

Fixes #12

• Dispatch the durable task worker from the Gateway scheduler.
• Recover running tasks and execute pending work in exact actor/workspace/session scopes.
• Keep failed and blocked tasks behind explicit resume.
• Add a real Uvicorn + temporary SQLite + file-effect integration test.

Validation:

• tests.test_durable_tasks_gateway passed with a real subprocess and file writes.
• make check passed.
• make lint passed.
• make test passed: 120 tests.
• GPG-signed PR head verified by GitHub.